[4.x] Allow custom form exporters to be defined

[ryanmitchell]

This PR allows custom form exporters to be defined in a new statamic.forms.exporters config key.

    'exporters' => [
        'csv' => [
            'class' => Statamic\Forms\Exporters\CsvExporter::class,
            'label' => 'Export as CSV',
        ],
        'json' => [
            'class' => Statamic\Forms\Exporters\JsonExporter::class,
            'label' => 'Export as JSON',
        ],
    ],

Say for example you wanted to add your own Excel exporter:

    'exporters' => [
        'csv' => [
            'class' => Statamic\Forms\Exporters\CsvExporter::class,
            'title' => 'Export as CSV',
        ],
        'json' => [
            'class' => Statamic\Forms\Exporters\JsonExporter::class,
            'title' => 'Export as JSON',
        ],
        'excel' => [
            'class' => App\Exporters\ExcelExporter::class,
            'title' => 'Export as Excel',
            'some_config_setting_i_need' => 100,
        ],
    ],

I've modified both the exporter contract and abstract class to allow config values to be set.

You can also specify a forms key in the config, and it will limit the exporter to those form handles.

Closes statamic/ideas#834

[marcorieser]

This is huge! It would be nice if there was an option to limit exports to specific forms.

[ryanmitchell]

@marcorieser sure, easily added. You specify a forms array to limit them to certain handles.

[jasonvarga]

Great idea. Just have a little different direction in mind.

[ryanmitchell]

All changes made as requested. I also made it possible in the CSV exporter to define csv_delimiter and csv_headers in the config for the exporter - so you could potentially change it per form if thats wanted. Maybe at some point those should be removed from the root level of the config.